Hedberg, Nils

Abstract [en]

Aquaculture is of growing importance in the global seafood production. The environmental impact of aquaculture will largely depend on the type of environment in which the aquaculture system is placed. Sometimes, due to the abiotic or biotic conditions of the seascape, certain aquaculture systems tend to be placed within or near specific ecosystems, a phenomenon that in this thesis is referred to as aquaculture system - ecosystem links. The exposed ecosystems can be more or less sensitive to the system specific impacts. Some links are known to be widespread and especially hazardous for the subjected ecosystem such as the one between the shrimp aquaculture and the mangrove forest ecosystem. The aim of this thesis was to identify and investigate causes and consequences of other spatial links between aquaculture and ecosystems in the tropical seascape.

Two different aquaculture system - ecosystem links were identified by using high resolution satellite maps and coastal habitat maps; the link between sea cage aquaculture and coral reefs, and the one between seaweed farms and seagrass beds. This was followed by interviews with the sea cage- and seaweed farmers to find the drivers behind the farm site selection. Many seaweed farmers actively choose to establish their farms on sea grass beds but sea cage farmers did not consider coral reefs when choosing location for their farms. The investigated environmental consequences of the spatial link between sea cage aquaculture and coral reefs were considerable both on the local coral reef structure, and coral associated bacterial community. Furthermore, coral reef associated fish are used as seedlings and feed on the farms, which likely alter the coral food web and lower the ecosystem resilience. Unregulated use of last resort antibiotics in both fish- and lobster farms were also found to be a wide spread practice within the sea cage aquaculture system, suggesting a high risk for development of antibiotic resistant bacteria. The effects of seaweed farms on seagrass beds were not studied in this thesis but have earlier been shown to be rather substantial within the borders of the farm but less so outside the farm.

Further, a nomenclature is presented to facilitate the discussion about production system - ecosystem links, which may also be used to be able to incorporate the landscape level within eco-certifying schemes or environmental risk assessments. Finally - increased awareness of the mechanisms that link specific aquaculture to specific habitats, would improve management practices and increase sustainability of an important and still growing food producing sector - the marine aquaculture.